The invention is based on an electromagnetically actuatable injection valve as defined hereinafter and to a method for producing a nozzle holder of an injection valve. German Patent 40 26 531.5 discloses an injection valve that has a valve closing member comprising a spherical valve closing body and an armature firmly connected to the valve closing body. The armature cooperates with a winding that is disposed on a core and through which current flows. The valve closing member is guided axially movable in a nozzle body, which is disposed in a nozzle holder bore of a nozzle holder. In the vicinity of the armature, the valve closing member is guided in a guide ring bore of a guide ring acting as an armature guide; the guide ring is disposed on a shoulder of the nozzle holder. The guide ring bore is embodied coaxially with the nozzle holder bore and guides the armature over its entire circumference.
German Offenlegungsschrift 39 25 212.4; U.S. application Ser. No. 508,630 filed Apr. 13, 1990, shows a similar arrangement, in which a valve closing member, comprising a spherical valve closing body, a connecting tube and an armature, is disposed in a nozzle holder bore of a tubular nozzle holder. The armature is guided over its entire circumference in a guide segment of the nozzle holder bore; this segment acts as an armature guide and is embodied coaxially with the nozzle holder bore on the upstream end of the nozzle holder. The guide segment has a smaller diameter than the nozzle holder bore. The connecting tube is firmly joined to the armature at one end and to the valve closing member at the other, so that when the winding has current flowing through it, the valve closing body lifts away from the valve seat face of the nozzle body and uncovers a narrow annular gap between the valve seat face and the valve closing body, through which the fuel flows in the direction of an injection port.
In both of the armature guides described above, guidance of the armature over its entire circumference produces strong frictional forces, because of the large area of contact between the armature and the guide ring or between the armature and the guide segment of the nozzle body bore; this makes fast motion of the valve closing member more difficult. The high frictional forces must be compensated for by using both a stronger restoring spring and a more powerfully dimensioned magnetic circuit.
To assure the axial mobility of the armature, the guide ring bore or the guide segment of the nozzle bore has a slightly larger diameter than the armature, so that in operation the armature can assume an eccentric position in the armature guide. An eccentric position of the armature leads to unilateral contact with the wall of the armature guide, producing a correspondingly larger gap on the opposite side. The uneven gap width over the circumference leads to nonhomogeneity of the magnetic field in the gap between the armature and the armature guide. The lack of homogeneity of the magnet field, and especially the contact of the armature on the armature guide, produce a lateral force toward the wall of the armature guide that increases the frictional forces between the armature and the armature guide still further. Guiding the armature in the guide ring bore or in the guide segment of the nozzle body bore is characterized by a narrow gap between the armature and the wall of the armature guide. This narrow gap seals off a first space, formed between the nozzle holder, the nozzle body and the armature, virtually completely from a second space located on the side of the armature toward the core. Upon each closing or opening movement, the armature is thus working against the volume of the space, which hinders the motion. The volume displacement work of the armature stands in the way of a fast motion of the valve closing member.
Moreover, guiding the armature by a guide ring inserted into the nozzle holder requires high production accuracy, since both the guide ring having the guide ring bore and the shoulder in the nozzle holder into which the guide ring is inserted must be manufactured with maximum accuracy. The use of high-precision production processes increases the effort and cost of production of the injection valve.